Organic light emitting diodes, which are a self-luminous device, are advantageous because of a wide viewing angle, high contrast, fast response time, high luminance, superior driving voltage, excellent response rate, and polychromatic properties.
Typical organic light emitting diodes are configured to include an organic emission material layer for emitting light, and a first electrode and a second electrode disposed on both sides of the organic emission material layer so as to face each other.
Such organic light emitting diodes are classified into a bottom emission type and a top emission type depending on the direction of light emitted from the organic emission material layer. A bottom emission type organic light emitting diode for emitting light to the substrate is configured such that a reflective electrode is formed on an organic emission material layer, and a transparent electrode is formed under the organic emission material layer. As such, when the organic light emitting diode operates in an active matrix mode, light does not pass through the portion thereof on which a thin film transistor is formed, thus reducing the area where light is transmitted. On the other hand, a top emission type organic light emitting diode is configured such that a transparent electrode is formed on an organic emission material layer and a reflective electrode is formed under the organic emission material layer, thus emitting light to a direction opposite the substrate, and thereby the area where light is transmitted is enlarged, resulting in high luminance.
The bottom emission type organic light emitting diode is configured such that an anode is formed on a substrate, and a hole transport layer, an emission material layer, an electron transport layer, and a cathode are sequentially formed on the anode. As such, the hole transport layer, the emission material layer, and the electron transport layer are organic thin films made of an organic compound.
The cathode comprises a metal layer having properties of a reflective layer, so that light generated from the emission material layer is reflected to the anode layer, thereby increasing luminous efficiency.
The driving principle of the organic light emitting diode thus configured is as follows. When voltage is applied between the anode and the cathode, holes injected from the anode are moved to the emission material layer via the hole transport layer, and electrons injected from the cathode are moved to the emission material layer via the electron transport layer. The carriers such as holes and electrons are re-combined in the emission material layer to form excitons. While these excitons return to a ground state from an excited state, light is produced.
As such, the generated light travels linearly to an anode direction, a cathode direction, and the other directions. The light traveling linearly to the anode is escaped to the air layer through glass, and the light traveling linearly to the cathode is reflected from the metal layer that is the cathode, and then goes again to the anode.
In this regard, Korean Patent Application Publication No. 10-2006-0095489 discloses an organic light emitting diode, which is configured such that an emission material layer is interposed between a first electrode and a second electrode, and a reflective layer for reflecting light emitted from the emission material layer to travel toward the second electrode is formed on the first electrode. Also, Korean Patent Application Publication No. 2001/0101640 discloses a technique for increasing luminous efficiency by determining the film thickness between a light-transmitting electrode and a reflective electrode so as to resonate the desired wavelength using interference caused by multiply reflecting light between the light-transmitting electrode and the reflective electrode.
FIG. 1 illustrates an organic light emitting diode manufactured by a conventional technique. Such a conventional technique is specified below. In order to manufacture an organic light emitting diode, a soda-lime or alkali-free glass substrate 10 is coated with a transparent conductive film 20 (ITO), after which a photoresist (PR) is applied thereon using a spin coater, followed by UV exposure, thereby forming a desired pattern. Thereafter, the device is loaded on a vacuum deposition machine, and a hole injection layer (HIL) 30, a hole transport layer (HTL) 40, an emission material layer (EML) 50, an electron transport layer (ETL) 60 and a cathode (a metal electrode) 70 are deposited.
Then when direct-current power or voltage ranging from ones to tens of V is applied to the transparent electrode and the metal electrode to allow current to flow, the organic light emitting diode emits light, and light irradiated toward the cathode is reflected through a reflective plate, and is then irradiated toward the glass substrate.
As such, the reflected light may exhibit an interference effect with light that travels toward the anode from the emission material layer, but conventional organic EL (electroluminescent) devices have low constructive interference effects due to structural limitation thereof, making it impossible to obtain high color coordinates. To obtain color coordinates corresponding to high color quality of the organic light emitting diode, proper color coordinates may be ensured by using a material having low color coordinates or by adjusting the device thickness, but driving voltage, efficiency and lifetime may deteriorate undesirably.
With the goal of solving the above problems, Korean Patent Application Publication No. 10-2014-0055911 (May 9, 2014), filed by the present inventors, discloses an organic light emitting diode configured such that a functional layer that enables mutual reinforcement and interference of transmitted light is formed on a light-transmitting electrode serving as the cathode, and a reflective layer is formed on the functional layer, thus realizing improved color coordinates and luminous effects using mutual reinforcement and interference of reflected light.
The above disclosure may be specified referring to FIG. 2. As illustrated in FIG. 2, the organic light-emitting display device using mutual reinforcement and interference of reflected light due to the presence of the functional layer comprises: a lower electrode 20 formed on a light-transmitting substrate 10; an organic thin film layer 30˜60 formed on the lower electrode 20 and including an emission material layer; a light-transmitting upper electrode 71 formed on the organic thin film layer; a functional layer 80 formed on the upper electrode 71 and enabling mutual reinforcement and interference of transmitted light; and a reflective layer 90 formed on the functional layer.
The operation of the organic light-emitting display device including the functional layer, as shown in FIG. 2, is described below in detail.
Specifically, when voltage is applied between the lower electrode 20, formed on the light-transmitting substrate 10, and the light-transmitting upper electrode 71, formed on the organic thin film layer, holes injected from an anode, i.e. the lower electrode, are transferred to the emission material layer via the hole transport layer, and electrons injected from a cathode, i.e. the upper electrode, are transferred to the emission material layer via the electron transport layer. The carriers, that is, holes and electrons, are recombined in the emission material layer to form excitons. When these excitons return to a ground state from an excited state, light is produced.
As such, the generated light travels linearly in the direction of an anode, the direction of a cathode, and other directions. The light traveling linearly to the anode escapes to the air layer through glass, and the light traveling linearly to the cathode passes through the light-transmitting upper electrode 71 and then through the functional layer 80, which is formed on the upper electrode and enables mutual reinforcement and interference of transmitted light, after which the light is reflected by the reflective layer 90 formed on the functional layer, and goes again toward the substrate via the functional layer and then via the upper electrode, and is thus emitted a predetermined period of time after the light traveling linearly to the anode.
As such, the light reflected by the reflective layer 90 causes constructive or destructive interference with the light that travels linearly to the anode by means of the functional layer and the organic thin film layer, the thickness of which is already adjusted, thereby controlling the spectrum characteristics. By virtue of the interference effect of light, the organic light emitting diode allows the optical spectrum of emitted light to have a sharp peak at a specific wavelength, thereby realizing an organic light emitting diode having high color quality and high efficiency.
However, in the case where the functional layer is formed on the upper electrode, the upper electrode must be formed thin to ensure high light transmittance. Accordingly, such an organic light emitting diode may be problematic in that the resistance of the upper electrode is high when manufactured to have a large area. As the upper electrode becomes distant from the electrode contact in the fabrication of a large-area organic light emitting diode, a large voltage drop may occur. Hence, solutions therefor are required.